In an aircraft gas turbine (jet) engine, air is drawn into the front of the engine, compressed by a shaft-mounted compressor, and mixed with fuel. The mixture is combusted, and the resulting hot combustion gases are passed through a turbine mounted on the same shaft. The flow of gas turns the turbine by contacting an airfoil portion of the turbine blade, which turns the shaft and provides power to the compressor. The hot exhaust gases flow from the back of the engine, driving it and the aircraft forwardly.
The hotter the turbine gases, the more efficient is the operation of the jet engine. There is thus an incentive to raise the turbine operating temperature. However, the maximum temperature of the turbine gases is normally limited by the materials used to fabricate the turbine vanes and turbine blades of the turbine. In current engines, the turbine vanes and blades are made of nickel-based or cobalt-based superalloys that can operate at temperatures of up to 1900-2100.degree. F.
Many approaches have been used to increase the operating temperature limits and operating lives of the airfoils of the turbine blades and vanes. The compositions and processing of the materials themselves have been improved. The articles may be formed as oriented single crystals to take advantage of superior properties observed in certain crystallographic directions. Physical cooling techniques are used. In one widely used approach, internal cooling channels are provided within the components, and cooler air is forced through the channels during engine operation.
In another approach, a protective layer in the form of an environmental coating or a ceramic/metal thermal barrier coating (TBC) system is applied to the airfoil of the turbine blade or turbine vane component, which acts as a substrate. One of the currently known protective layers is a diffusion aluminide layer. A diffusion aluminide protective layer may be formed, for example, by electrodepositing a layer of platinum onto the surface to be protected, depositing a layer of aluminum over the platinum layer, and interdiffusing the two deposited layers.
This protective layer, with no overlying ceramic layer, is useful in intermediate-temperature applications. For higher temperature applications, a ceramic thermal barrier coating layer may be applied overlying the protective layer, to form a thermal barrier coating system. The ceramic thermal barrier coating layer insulates the component from the exhaust gas, permitting the exhaust gas to be hotter than would otherwise be possible with the particular material and fabrication process of the substrate.
Platinum is expensive, and therefore care is taken not to deposit more platinum than necessary. However, in existing practice excessive platinum is still used. There is a need for an improved approach to the preparation of airfoils with a platinum aluminide protective layer, which reduces the use of platinum. The present invention fulfills this need, and further provides related advantages.